1. Field of the Invention
The present invention relates generally to a method for determining the state switching time for intermittent recording in time-lapse video tape recorder. More specifically, the present invention relates to a method for determining when magnetic tape starts to be reloaded or head drum does to be rotated to resume to record intermittent video frames which have been stored in memory after recording pause period in time-lapse video tape recorder.
2. Brief Description of the Prior Art
Unlike an ordinary continuous video tape recorder which records every frames of video signals, time-lapse video tape recorder intermittently records video signals from several video cameras or the like on a magnetic tape with a pre-selected regular interval while the recorder repeats running and stopping. It has been used in various areas requiring long-time recording such as a security monitoring system in banks and museums. As shown in FIG. 1, most time-lapse video tape recorders comprise an A/D converter 100 which converts analog video signal from an external video camera into a digital data stream, a buffer memory 201 in which the converted digital video data are stored temporarily; a field memory 202 in which the video data from the buffer memory 201 are stored before they are recorded, a D/A converter 300 which converts the digital video data in the field memory 202 into analog video signal, a recording unit 400 which records the converted analog video signal on the magnetic tape; and a controller 500 which controls writing/reading intervals of video signal from the external video camera to/from memories 201 and 202 according to the command through an input unit 600 from a user.
The recording unit 400 comprises a signal processing unit which processes the video signal for recording; and mechanical elements, shown in FIGS. 2 and 3 such as a loading motor 401 which moves a loading post 1 to load/unload a magnetic tape; a drum motor 402 which rotates a head drum 3 on which video heads 2 are mounted to record/reproduce video signal on the magnetic tape; a capstan motor 403 which rotates a capstan shaft 4 to drive the magnetic tape at a designated tape running speed.
In a conventional time-lapse video tape recorder configured as above, one frame is extracted from continuous video signal from an external video camera at intervals of a predetermined time and is then stored temporarily in the buffer memory 202 after it is digitized by the A/D converter 100. Then, the video frames in the buffer memory 202 are transferred into the field memory 202. This way, continuous video signal is compressed into the digital video signal which is sub-sampled by frame.
On the other hand, when recording pause period, or duration in which no video frame is recorded (duration in which the video frames are stored in the field memory 202), is longer than a reference period of time, the state of the recording unit 400 is changed such that the magnetic tape loaded (FIG. 2) is unloaded (FIG. 3) or the rotation of the head drum 3 is stopped temporarily during the recording pause period.
When the field memory 202 is filled maximally with the extracted video frames, they are retrieved at a time by the controller 500 and are then converted into an analog video signal by the D/A converter 300. The analog video signal begins to be recorded after reloading the tape that has been unloaded during the recording pause period or resuming the rotation of the head drum 3 that has been stopped during the recording pause period. While the video frames in the field memory 202 are recording on the magnetic tape, new video frames are sampled intermittently from continuous video signal and stored in the buffer memory 201 temporarily. As soon as the recording of the video frames in the field memory 202 is completed, the video frames which have been 20 stored in the buffer memory 201 are transferred to the field memory 202, along with video frames which are newly stored in the buffer memory 201 after the completion of the recording operation. Whether or not the video frames stored newly in the buffer memory 201 after the recording operation exists depends on the capacity of the buffer memory 201.
In case of the prior art above, some action is not made to resume recording such as loading of the magnetic tape that has been unloaded during the recording pause period or rotating the head drum 3 that has been stopped during the recording pause period at a normal rotation speed till the field memory 202 is full, or the number of the video frames becomes equal to the capacity of the field memory 202.
Therefore, as shown in FIG. 4, the video frames in the field memory 202 cannot be recorded until the stable recording operation is possible since loading of the magnetic tape. In other words, it is possible to record the video frames stored in the field memory 202 since 4 seconds or so elapses which is required to load the magnetic tape that has been unloaded by driving the loading motor 401 and to make the recording operation stable. Likewise, when the head drum stopping scheme is used in the recording pause period, recording of the video frames is not allowed till the head drum 2 begins to rotate at the normal speed, which usually takes 4 seconds or so.
Because of this additional time for preparation of stable recording, the period in which video frames are kept temporarily in the buffer memory 201 is longer than actual recording time of the video frames in the field memory 202. As a result, the size of the buffer memory 201 should be large enough to accommodate the video frames received from an external video camera during a recording preparation period in which the recorder is ready for recording.
The primary objective of the present invention is to solve the above mentioned problems in the conventional method and to provide a method for determining when magnetic tape/head drum starts to be reloaded/rotated to resume recording before memory becomes full with intermittent video frames in time-lapse video tape recorder.
The present invention is characterized as a method for determining when magnetic tape/head drum is reloaded/rotated to resume recording in time-lapse video tape recorder which comprises the three steps: calculating an amount of memory available for temporary storing video frames while storing the video frames received from external video camera in the memory in the intermittent recording mode; continuing to store the video frames in the memory until the amount of available memory calculated is smaller than a predetermined amount; and either reloading the magnetic tape unloaded or rotating the head drum stopped when the amount of available memory becomes equal to the predetermined amount.
According to the present invention, in the method for determining when magnetic tape/head drum is reloaded/rotated to resume recording in time-lapse video tape recorder, a frame extraction ratio of video signal, or a ratio of the number of frames to be skipped and the number of frames to be recorded, is calculated from a recording time in a time-lapse recording mode that is set by a user. Based on the frame extraction ratio and the capacity of the memory in which the extracted video frames are temporarily stored, the remaining storage time of the memory is calculated which corresponds to the amount of the memory available for temporary storing.
When the remaining storage time of the memory becomes smaller than a predetermined reference period of time, before the memory is full the controller 500 executes commands to restart recording operation such as loading of the magnetic tape unloaded or rotating of the head drum stopped. Thus, when the number of video frames becomes the capacity of the memory, it is possible to record all the video frames that have been maximally stored in the memory without some time delay.
The above objectives, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof to read in conjunction with the accompanying drawings.